Method for dissolving salts in 1,2-dichlorethane using ultrasound and a device for carrying out said method

ABSTRACT

The invention relates to a process and a device for the dissolution of salt that is hardly soluble, especially sodium chloride and other poorly soluble salts in 1,2 dichloroethane, which primarily are to be used in direct chlorination plants for the production of 1,2 dichloroethane. This aim is achieved by mounting an ultrasonic transducer (sonotrode) in the dissolution chamber which is filled with a suspension of salt crystals and 1,2 dichloroethane. The suspension is sent through a filter upon dissolution of the salt.

[0001] The invention relates to a process for the dissolution of salts,especially catalytically active sodium chloride NaCl and ferric chlorideFeCl₃ in liquid 1,2 dichloroethane, hereinafter referred to as “EDC”,which primarily serves as an intermediate product in the production ofmonomer vinyl chloride, hereinafter referred to as “VCM”, whichsubsequently is used to produce polyvinyl chloride (PVC) by ultrasonictreatment. When EDC reacts to form VCM hydrogen chloride HCl isobtained. Hence, EDC is preferably produced from ethene C₂H₄ andchlorine Cl₂ in such a way that an equilibrated balance is maintainedbetween the hydrogen chloride (HCl) produced and consumed in the variousreactions, which is substantiated by the following reaction equations:

Cl₂+C₂H₄→C₂H₄Cl₂(EDC)+180 kJ/Mol  (1)

C₂H₄Cl₂(EDC)→C₂H₃Cl(VCM)+HCl−71 kJ/Mol  (2)

C₂H₄+2HCl+½O₂→C₂H₄Cl₂(EDC)+H₂O+238 kJ/Mol  (3)

[0002] Reaction (1) also referred to as direct chlorination is usuallycarried out as gas/liquid interfacial reaction in loop reactors ofvarious designs, which provide for the dissolution of ethene C₂H₄ andthus for the speed-relevant criterion, or it takes place in the liquidphase. The EDC is applied as the solvent used for the dissolution of thereactants. Many years of practical experience have shown that the purityof the EDC produced is of major importance for the cost effectivenessand the final product purity which can be achieved by reaction (2) andconsequently for the complete process. This gave way to many attempts tominimise the side reactions related to reaction (1) as, for example, bythe development of efficient catalyst systems.

[0003] Reaction (1) is of the “addition type”, in which generally metalhalides with Lewis acid properties and a metal halide of a metal thatbelongs to the first main group of the periodic table are used ascatalyst as well as other organic catalysts if necessary. NL-A 6901398provides for the use of ferric chloride FeCl₃, sodium chloride NaCl andlithium chloride LiCl as preferred metal halides. DE 41 03 281 describesthe use of a mixture of ferric chloride FeCl₃ and sodium chloride NaClat a molar ratio ranging from 1 to 1.5 and 1 to 2. DE 43 18 609, incontrast to this, outlines that the reaction is especially efficient ifthe molar ratio of NaCl and FeCl₃ is kept below 0.5. In these cases,sodium chloride NaCl reacts with ferric chloride FeCl₃ in solution byreaction (4), thus forming sodium tetrachloroferrate NaFeCl₄ which hasbeen known to be an efficient catalyst for a long time:

Na⁺Cl⁻+FeCl₃→Na⁺FeCl₄ ⁻  (4)

[0004] In this context, however, it is problematic that sodium chlorideNaCl is hardly soluble in EDC. This also applies to the fact that undernormal reaction conditions and in the presence of minor amounts ofwater, ferric chloride FeCl₃ tends to convert to hydrogentetrachloroferrate which is known for its highly corrosive potential asdescribed in EP 00 82 342. Such a corrosive potential can be suppressedby adding sodium chloride NaCl in a stoichiometric surplus referred toreaction with ferric chloride FeCl₃ according to the equation (4) sothat it becomes desirable to dissolve as much sodium chloride NaCl aspossible. If, however, solid sodium chloride NaCl is added to thereaction loop, solute may easily form in sections with poor flow andwill not dissolve even under normal operating conditions of the directchlorination unit and thus give way to side reactions.

[0005] As described in patent DE 25 40 291 further difficulties may alsobe caused by clogging as a result of inefficient solution of the sodiumchloride NaCl. A specific attempt has also been described, i.e. applyinginstead anhydrous sodium tetrachloroferrate NaFeCl₄ which according toequation (4) consists of the sum of sodium chloride NaCl and ferricchloride FeCl₃ to be dissolved in EDC because its solubility is betterthan that of its components. In this case, however, the technologicalproblem is shifted to the production of sodium tetrachloroferrateNaFeCl₄ which is very expensive according to U.S. Pat. No. 3,729,543.

[0006] Various attempts in the past have shown that methods to producethe preferred catalyst system are very sophisticated because thecatalysts can be dissolved in the EDC reaction system with greatdifficulty only. DE 44 25 872 describes an example on the basis of asolution of only 170 ppm sodium chloride NaCl and 780 ppm ferricchloride FeCl₃.

[0007] The aim of the invention therefore is to dissolve salt,especially sodium chloride NaCl and ferric chloride FeCl₃, in afavourable manner and amount that preclude any disadvantage that wouldelse occur during operation as a result of solute accumulation, inparticular non-dissolved sodium chloride NaCl, and that eliminate anydisadvantage coming up when a change from sodium chloride to any other,more readily soluble catalyst takes place in the direct chlorinationprocess for EDC production; moreover that aim is to overcome anydisadvantage that is normally linked to the external preparation ofeasily soluble catalysts.

[0008] It was a real surprise to find that the amount of sodium chlorideNaCl that can be dissolved in EDC which already contains ferric chlorideFeCl₃ in solute form is considerably larger than that hitherto achievedin production plants, provided the dissolution process is supported byultrasonic treatment.

[0009] The process according to the invention achieves the aim definedin the previous paragraphs by providing an ultrasonic treatment of thesuspension of solid, granular sodium chloride NaCl in liquid EDC whichcontains ferric chloride FeCl₃ in solute form and by providing for asubsequent filtration of said suspension.

[0010] The ultrasonic treatment generates an effect that prevents thecrystal surfaces from being covered with a layer of higher molecularby-products originating from the direct chlorination reaction and beingcirculated in the reaction loop, that enhances the mass transfer andthat precludes crystal agglomeration. This permits an efficientdissolution of the catalyst components so that no suspended solidspenetrate the reaction loop.

[0011] An embodiment of the invention process provides for ferricchloride FeCl₃ and sodium chloride NaCl to jointly suspended in EDC. Thesubsequent ultrasonic treatment has the effect that ferric chlorideFeCl₃ will dissolve first and that upon dissolution of sufficient ferricchloride, sodium chloride NaCl will dissolve in the EDC which at thisstage already contains ferric chloride in solute form.

[0012] The clear solution obtained, which has a molar ratio of ferricchloride FeCl₃ and sodium chlorite NaCl of optionally 1:1, is admixed tothe reaction fluid. The process according to the invention is suitableeither for continuous operation in a side stream of the plant or forbatch operation. The addition of sodium chloride NaCl to EDC and itsdissolution by ultrasonic treatment may be implemented either byseparate devices or by one common device.

[0013] The details of the invention are illustrated in the attachedfigures Nos. 1 and 2.

[0014]FIG. 1 schematic layout of process according to the invention

[0015]FIG. 2 schematic layout of a preferred design of the devicerequired to implement the process

[0016]FIG. 1 shows the example of a dissolution device 1 for sodiumchloride in conjunction with loop reactor 11, the sodium chloridesuspension 2 being fed to dissolution device 1. As an option, sodiumchloride NaCl may also be fed as bulk through an opening in or a hopperof dissolution device 1, but in this case it is essential to precludeany escape of EDC vapour because it is extremely toxic. Liquid EDC 3 isfed as solvent to dissolution device 1. Dissolution device 1 houses anultrasonic transducer, hereinafter referred to as sonotrode 4 andconnected to ultrasonic generator 5. Said sonotrode 4 emits theultrasonic waves into the suspension. Solution 6 leaves the dissolutiondevice 1 upon filtration in device 7.

[0017] Apart from the example of sodium chloride NaCl, it is alsopossible to dissolve other salts by this method, which in particularapplies to metal chlorides of the first main group of the periodicsystem (alkaline metals) and to ferric chloride FeCl₃. It is alsopossible to dissolve salt mixtures by this method.

[0018] Said solution 6, for example, is directly injected into loopreactor 11 but without abandoning the basic idea of the invention, it isalso feasible to arrange the feed point in a more favourable position inthe EDC loop. Loop reactor 11 consists of chlorine dosing device 8,chlorine dissolution section 9, riser 10, downcomer 12, ethene additionfacility 13 and stripper 14, the EDC produced being withdrawn either asvaporous EDC 15 or liquid EDC 16. Upon heat recovery a part stream ofthe EDC product stream can, for example, be re-used to dissolve sodiumchloride NaCl in dissolution device 1 but without abandoning the basicidea of the invention, it is also feasible to use any other EDCpart-stream of the EDC loop. The higher the temperature of thatpart-stream, the better it will be to carry out the dissolutionaccording to the invention.

[0019]FIG. 2 shows a preferred embodiment with dissolution chamber 17,cylindrical sonotrode 18 and filtration device 19. The said assembly issuitable for batch service and for continuous operation. To feed salt tothe unit it is possible to mount a feed hopper for salt (not shown inthe layout) to nozzle 20. The EDC solvent is fed to dissolution chamber17 via feed line 21 with integrated strainer 22, the solution beingdischarged via line 23. The suspension of the fed salt is also containedin said chamber 17 which is filled with liquid EDC at least up to thelevel of the discharge line and blanketed with inert gas from the gaschamber arranged above. Any salt particles contained in dissolutionchamber 17 are entrained by the stream and conveyed to filtration device19 where they are retained by the filter cake. Said filter cake and thesuspension in the section surrounding the cake are subjected toultrasonic treatment by means of the sonotrode so that the passivatedlayers which form on the salt crystals are continously removed and thepoorly soluble salt will dissolve. It is recommended to arrange thefiltration device in such a manner that a large surface area surroundsthe sonotrode and that the distance from the filtration device to thesonotrode is sufficiently large to accommodate the filter cake and topreclude obstruction of the stream on the one hand, and that saiddistance is only a few centimetres to permit free propagation of theultrasonic waves to the filter cake, on the other hand.

[0020]FIG. 2 also shows drain line 24 which is normally closed andultrasonic generator 5 required to operate cylindrical sonotrode 18. Thelayout does not show the safety shut-off devices, the blanketing devicenor the explosion-proofing equipment which the EDC specialist will ofcourse install to meet such requirements.

REFERENCE LIST OF DESIGNATIONS

[0021]1 Dissolution device

[0022]2 Suspension of sodium chloride

[0023]3 EDC

[0024]4 Sonotrode

[0025]5 Ultrasonic generator

[0026]6 Solution

[0027]7 Filtration device

[0028]8 Chlorine dosing device

[0029]9 Chlorine dissolution section

[0030]10 Riser

[0031]11 Loop reactor

[0032]12 Downcomer

[0033]13 Ethene addition facility

[0034]14 Stripper

[0035]15 Vaporous EDC

[0036]16 Liquid EDC

[0037]17 Dissolution chamber

[0038]18 Cylindrical sonotrode

[0039]19 Filtration device

[0040]20 Feed nozzle

[0041]21 Feed line

[0042]22 Strainer

[0043]23 Discharge line

[0044]24 Drain line

1. Process for the dissolution of salts in liquid 1,2 dichloroethane,characterized in that a suspension of liquid 1,2 dichloroethane and saltparticles undergo ultrasonic treatment.
 2. Process according to claim 1,characterized in that the dissolved salt is used as a catalyst for theproduction of 1,2 dichloroethane in a direct chlorination plant. 3.Process according to one of the preceding claims 1 to 2, characterizedin that the suspension which undergone ultrasonic treatment is filtered.4. Process according to one of the preceding claims 1 to 3,characterized in that the salt to be dissolved at least consists partlyof an alkali chloride.
 5. Process according to claim 4, characterized inthat the salt to be dissolved is sodium chloride.
 6. Process accordingto one of the preceding claims 1 to 3, characterized in that the salt tobe dissolved at least consists partly of an ferric chloride.
 7. Devicefor running the process according to claim 1, characterized in that theultrasonic treatment of the suspension is provided by an sonotrodeimmersed in the dissolution chamber.
 8. Device for implementing theprocess according to claim 7, characterized in that the dissolutionchamber houses a filtration device between the sonotrode and thedischarge end for the solution.
 9. Device according to claim 8,characterized in that said filtration device is arranged in such amanner that a filter cake forms and is directly treated by ultrasonicwaves emitted by the sonotrode.